1. Field of the Invention
This invention relates to a method and apparatus for measuring scalefactor variation in a vibrating structure gyroscope.
2. Discussion of Prior Art
There are many known implementations of vibrating structure gyroscopes (VSG). These make use of a wide variety of resonant structures including vibrating beams, tuning forks, hemispherical shells & planar rings. These may be fabricated using numerous techniques using a range of materials. Structures fabricated from silicon using micro-machining techniques have the particular advantage that they may be produced in large numbers and at low cost whilst maintaining a high degree of performance. This makes them particularly suitable for applications demanding high volumes of low cost sensors. One such application is for yaw rate sensing in advanced braking systems for automotive vehicles.
Many of the applications for these sensors are critical to the safe operation of the vehicle. The integrity of the gyroscope output is of paramount importance as it is used to control the functionality of such systems. In the advanced braking system application this signal is used to control the application of the brakes. System designers therefore require a very high degree of Built in Test (BIT) coverage on such sensors. Techniques for testing the basic functionality of such devices are known, and disclosed, for example in U.S. Pat. No. 5,866,796A and U.S. Pat. No. 5,889,193A. While these known techniques provide a good level of coverage they are not able to test for failures associated with scalefactor (the scaling, that is change, of the sensor output in response to a given applied rate of rotation). Such errors can result in the rate measurement indication of the sensor being significantly different from the true rotation or even absent entirely in some circumstances. This can result in the system failing to operate or operating erroneously with potentially disastrous consequences.
There is thus a need for a means or technique for providing BIT coverage of accurate scalefactor operation of VSG sensors. Preferably this functionality should be provided without the requirement for additional hardware and without additional expense.